This invention relates generally to processing image data obtained by scanning a picture, document or other image and, more particularly, to determining whether portions of the image represented by the image data correspond to text or continuous tone such that the image data may be appropriately processed for storage, duplication or display of the image.
Image data is obtained by scanning an image with a scanner comprising, for example, a plurality of charge coupled devices (CCD's). The scanner effectively divides the image into a finite number of small picture elements which are referred to as pixels or pels. Each resulting pel of the image is converted into a number representative of the grayscale value of the pel as detected by one of the scanner CCD's.
The image data is often applied to a bilevel device, i.e., a device which reproduces each pel as one of two grayscale levels, typically black or white, for duplication or display of the image. For application in a bilevel device, the image data representative of each pel is processed into a binary number signifying whether the pel is to be black or white, e.g., a print or no print picture element, respectively, for an image duplication system. The determination of which pels of image data are to be printed or left not printed for a bilevel image duplication system depends to some extent on the characteristics of the image to be duplicated.
For example, portions of the image may be broadly classified as comprising either continuous tone or text. Photographs and certain half tone images are examples of continuous tone while text is exemplified by line drawings and letter images. Different techniques are normally applied to process the two different types of image data.
For text image data, the conversion from grayscale values to binary values is often accomplished by establishing a threshold to which the grayscale value of each pel is compared. The result of the comparison is that if the grayscale value exceeds the threshold, a black or print representation is selected for the pel and, conversely, if the threshold is not exceeded, the pel is left white or not printed.
For continuous tone image data, alternate techniques are applied. For example, a variety of pel block patterns may be selected to represent blocks of the continuous tone image data dependent upon the composite grayscale value of the blocks. For continuous tone image data, pel block patterns may be shifted, rotated or otherwise varied to prevent the appearance of interference patterns such as Moire patterns.
One known prior art technique for distinguishing between text and continuous tone image data employs an electronic filter. The filter is applied to a block of pel data surrounding a particular pel to be evaluated. The difference between the grayscale value of the pel being evaluated and the average filtered grayscale value for the block of pels is determined. If the absolute value of the difference is above a preset threshold, text image data is presumed; and, if the absolute value of the difference is below or equal to the preset threshold, continuous tone image data is presumed. This technique is more fully disclosed in U.S. Pat. No. 4,194,221.
While the known prior art arrangement provides a varying level of effectiveness dependent upon the selection of coefficients for use in the electronic filter, improved techniques for distinguishing between text and continuous tone image data are always needed to advance the art of image processing and provide effective and inexpensive alternatives.